Technical Abstract:
Effective lubrication in a dynamic mechanical system is primarily governed by the formation of a stable tribochemical film as well as a thin (typically monomolecular) layer of lubricant molecules between the moving metal parts. Polar functional groups in the triacylglycerol molecules of vegetable oil in conjunction with oil-additive-metal interaction during the metal rubbing process can significantly improve the wear resistance and extreme pressure lubrication properties. The long fatty acid chain and presence of polar groups in the vegetable oil structure makes it amphiphilic in nature, therefore allowing them to be used as both boundary and hydrodynamic lubricants.
Increasing the polar functionality of the vegetable oil structure has a positive impact on wear protection resulting from stronger adsorption on metal surface as well as greater lateral interaction between the polar heads and long hydrocarbon chains of the ester group. Triacylglycerol molecules orient themselves with the polar ends directed towards the metal surface making a close packed monomolecular or multimolecular layered structure resulting in a surface film believed to inhibit metal-to-metal contact and the progression of pits and asperities on the metal surface.
Strength of the fluid film and extent of adsorption on the metal surface dictate the efficiency of the lubricant's performance, and it has also been observed that the friction coefficient and wear rate are dependent on the adsorption energy of the lubricant. During the tribochemical process, phosphorus containing additive molecules undergo significant chemical transformation in the presence of fatty acids and enhance the antiwear properties of vegetable oils and their derivatives.
The results revealed that greater availability of ester moieties resulted in the formation of a phosphate-ester based protective layer on the wear track, which can effectively function as friction modifiers in the system. Antimony impregnation into the metal surface following a strong complexation reaction with metal sulfide results in significant improvement in extreme-pressure characteristics due to alteration in surface metallurgy.
In this case, seizure is averted even when the fluid adsorbed layer and phosphite coating is removed because of antimony induced surface hardness. The difference can be observed by analyzing the metal surface using a scanning electron microscope.
Selected bio fluids viz. soybean oil, thermally modified soybean oil and chemically modified soybean oil were investigated for potential application as lubricants and industrial fluids.